This invention relates to liquid crystal display systems and particularly to such systems which comprise an array of individually addressable electrodes formed on a semiconductor backplate.
One type of liquid crystal display systems comprises a plurality of individually addressable cells arranged in a matrix array, with each of the elemental cells of the array operating as a reflective light valve. In the absence of applied potential, the liquid crystal material is clear and the cell appears dark to an observer. When electric potential above a threshold level is applied across the liquid crystal material, it scatters the light, much like a piece of frosted glass, and the cell appears white to the observer. The percentage of incident light which is scattered towards the viewing area is proportional to the magnitude of the potential applied to the liquid crystal cell, and consequently gray level display presentations may be produced.
In high resolution display systems a composite presentation is built up from thousands of individually controllable elemental liquid crystal cells, each of which must be updated (the potential across the cell reprogrammed) at a rate sufficiently high to prevent observable flicker in the presentation. This may be accomplished by sandwiching a thin layer of liquid crystal material between a glass plate having a transparent electrode, and a backplate having a matrix array of reflective electrodes formed on a semiconductor wafer. The backplate also contains the individual addressing circuitry (field effect transistor) and electrical storage circuitry (capacitor) disposed contiguous to the reflective liquid crystal contact electrode for each cell, with the reflective electrode forming one element or plate of the capacitor. Each field effect transistor is utilized to address an associated element of the matrix array, and the associated storage capacitor maintains the applied potential across the liquid crystal cell until the information is updated (every 33 milliseconds in a typical display). An example of this type of construction is disclosed and claimed in U.S. Pat. No. 3,862,360 issued to Dill et al, and assigned to the assignee of the present invention.
Liquid crystal display systems of the type just described are reflective displays and therefore require incident light for their operation. Unfortunately, the field effect transistors (FET's) used to control the storage capacitors in the backplate are extremely sensitive even to very low levels of incident light. As a result the ability of the storage capacitors to maintain the applied potential across the liquid crystal cells between pulses is seriously affected. The shielding effect of the reflective electrodes which lie in a plane between the front of the display and the field effect transistors is insufficient to prevent incident light from discharging the potential on the storage capacitors. This is so because there must be a gap between respective columns and rows of electrodes.